1. Field of the Invention
This invention relates to a semiconductor device and more particularly to a MIS field effect transistor having a source and drain improved and a manufacturing method thereof.
2. Description of the Related Art
In order to enhance the performance of a semiconductor integrated circuit, it is necessary to enhance the performance of field effect transistors which are the constituent elements thereof. The guiding principle for the improvement of the performance of the elements is based on scaling and the element performance has been enhanced by miniaturizing the elements so far. However, the limitation of miniaturization in future is pointed out and formation of shallow junction becomes a serious problem. At present, a solution for formation of 10 to 20 nm junction (drain-extension portion) of the 65-nm generation cannot be given according to the international semiconductor road map.
In recent years, a MOSFET having a source and drain formed to form Schottky junction instead of the conventional pn junction is proposed (J. R. Tucker et al., Appl. Phys. Lett., vol. 65, no. 5, August 1994, pp. 618-620). According to the document, since source and drain portions are formed of metal, diffusion of impurity is not utilized and extremely shallow junction can be formed. Further, various advantages that parasitic resistance can be lowered since the resistance of metal itself is extremely low, an ion-implantation process can be omitted to simplify the process and the like can be attained. Thus, the above MOSFET is expected as a field effect transistor of next generation.
In this type of field effect transistor with the Schottky junction, an effect that the parasitic resistance of the source and drain portions is lowered and the short channel effect is suppressed can be attained. However, since the field effect transistor utilizes the Schottky junction, there occurs a problem that a leak current becomes large.
Thus, in the conventional case, in order to lower the parasitic resistance of the source and drain portions and suppress the short channel effect, the field effect transistor having the source and drain formed to form the Schottky junction is effective. However, in this type of Schottky transistor, there occurs a problem that a leakage current becomes large.
More specifically, in this type of Schottky transistor, the Schottky junction of the source and drain portions determines the characteristic of the transistor. Therefore, in order to realize a preferable characteristic, it is necessary to sufficiently lower the height of a Schottky barrier of metal used to form the source and drain portions in comparison with that of carriers. However, the Schottky barrier height is originally and substantially determined by the type of metal and it is difficult to freely control the Schottky barrier height.
To control the Schottky barrier height as is desired, it is proposed that a source-drain impurity layer to be bonded to the Schottky source and drain. The resultant structure is nothing more than a transistor that has a pn junction used as source and drain. The structure can no longer retain the characteristics of a Schottky transistor that suppresses short-channel effect.